Slurry for centrifugal classification of colloidal particles

ABSTRACT

Removal of classified particles from the classifier apparatus is simplified by the addition of a polymeric dispersant in the slurry, added in an amount substantially in excess of that needed to disperse the particles.

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 028,891, filed Mar. 23, 1987, now U.S. Pat. No.4,781,671, which is hereby incorporated herein by reference.

FIELD OF INVENTION

This invention relates to a process for centrifugal classification ofpowders and in particular, to removal of those powders from thecentrifuge bowl.

BACKGROUND OF THE INVENTION

There are numerous advantages in using narrow size range ceramic powdersas starting materials, including uniform microstructures leading tooptimal performance properties. One method of producing narrow sizematerials is by a centrifugal classification system, described in theabove application. One of the problems encountered in making narrow sizerange powders in this fashion, is that the resulting powder is dilatentand quite difficult to remove from the centrifuge bowl, often requiringthe powder from being chiseled out in a batch mode. The dilatent aspectof the powder prevents continuous classification to be carried out usingconventional equipment, such as an auger-type implement which emptiesthe bowls.

DISCLOSURE OF THE INVENTION

I have found that by providing a substantial excess in the amount ofdispersant necessary for dispersion of particles in the feed slurry, thecolloidal dispersion of the feed material remains stable and theclassified narrow size range particles form a plastic rather thandilatent cake in the centrifuge bowl. Unlike a dilatent cake, this canbe handled easily and can be scooped out of the centrifuge bowl ratherthan forced out. The plastic character of the cake formed in accordancewith this invention allows a centrifugation classification system tooperate continuously and efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of the centrifugal classification processaccording to the present invention.

FIG. 2 is a scanning electron micrograph of 0.4-0.7 um size particlesclassified in accordance to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be applied to any type of colloidal particlewhich one wishes to classify by centrifugation techniques. Preferredembodiments use metallic particles and ceramic particles, particularly,but not limited to, alumina, yttria, silicon nitride, forsterite,mullite, cordierite, partially stabilized zirconia, and silicon carbide.

In the past, powder would be mixed with liquid to produce a relativelydilute but dispersed feed. However, the particles in this mixture lackeda lubricant and the collected centrifuge cake was dilatent uponclassification and subsequent collection. Addition of plasticers orlubricants such as humectin , for example, is not preferred becausethese agents may tend to cause flocculation, ruining the dispersion ofthe particles in the slurry, or may segregate non-uniformly during thecontinuous classification process.

In accordance with this invention, it has been found that theintroduction of an excess of a polymeric dispersant allows the particlesto remain dispersed prior to classification, yet provides the neededlubrication to allow them to flow after classification. The polymericdispersants useable is this invention can be virtually any commerciallyavailable one. Preferred dispersants include, but are not limited to:NARLEX LD-45 (an ammonium acrylate copolymer, available from theNational Starch Co., Bridgewater, N.J.) which is especially preferredwith alumina powders classified in water, and Darvan C and Darvan 821A(respectively, an anionic polyelectrolyte of the ammonium salt type andan anionic ammonium polyacrylate, both available from R.T. VanderbiltCo., Norfolk, Conn.).

The amount of dispersant added will vary with the type of dispersant andpowder. It should be enough to impart plasticity to the classified cake,but not be so much as to interfere with flow or dispersioncharacteristics of the slurry as it is classified.

The above invention may be better understood with reference to thefollowing examples, which are not intended to limit the scope of thisinvention.

EXAMPLE 1

A two stage classifying system was first primed with one residencevolume of water. Al₂ O₃ (60 weight percent) A-16 Superground fromAluminum Company of America, Pittsburgh, Pa., was mixed with water(39.25 weight percent) and the dispersant, a polymeric polyelectrolytesold under the brand name Narlex LD-45 (0.75 weight percent suspensionbasis). This mixture was milled, filtered, then pumped into theclassified feed tank. The feed suspension was then pumped at 0.85 kg/mininto the first stage classifier, operating at 1268 RPM which resulted inthe removal of the particles larger than 0.8 micrometers from the feedstream. The particles were collected in a bowl.

Suspension containing particles smaller than 0.8 micrometers was pumpedat 0.760 kg/min into the second stage classifier operating at 1942 rpm.Here a separation of particles of 0.5-0.8 micrometers with a specificsurface area of 9.6 m² /g±0.5 m² /g, calculated for over 150 productionruns was achieved.

In all these narrow sized classified cakes, the powder formed a dilatentsediment which was very difficult to remove from the centrifuge bowl. Ahammer and chisel proved most effective for powder removal after dryingthe cake and bowl together at 200° C. for 30 minutes.

EXAMPLE 2

A stock suspension of the following materials was made and milled for 24hours on a roller mill:

4.0 Kg A16 SG alumina (Aluminum Co. of America, Pittsburgh, Pa.)

3.6 Kg deionized water

50 g LD-45

A feed suspension was made, starting with 1.0 Kg of the stock suspensionand diluted with 2.2 Kg of distilled water and 800 g LD-45 solution. Thefeed suspension was milled for 30 minutes, filtered, and pumped into aclassifier feed tank similar to that described in the above-referencedpatent application Ser. No. 028,891, now U.S. Pat. No. 4,781,671, andprocessed as shown in FIG. 1.

The feed was pumped at 0.3 Kg/minute into a first classifier operatingat 3120 rpm. The fines overflow from the first classifier was pumped at0.17 kg/MIN into a second classifier operating at 4000 rpm. Finesoverflowing this classifier were discarded.

Adding the additional dispersant causes an increase in solutionviscosity from 1.1 to 2.4 cP, measured at 100 sec⁻¹.

Equation 1 below predicts the particle size cut in second bowl: ##EQU1##where v=a particle's settling velocity, h=the distance through which theparticle settles, t=the time required for the particle to settle throughdistance h, r=the particle radius, g=acceleration due to gravity, ρ_(p)=particle density, ρ_(m) =density of the medium, η=liquid viscosity, andK=the particle-shape factor (2/9 for a sphere), which takes into accountboth a particle's volume and its cross-sectional area. This equationpredicts an average equivalent spherical diameter particle size fractionof from about 0.4 to 0.8 micrometers, which was verified by scanningelectron microscopy (shown in FIG. 2).

The narrow size range powder collected in the bowls was not dilatent.Rather, it was plastic and could easily be scooped out of the bowl.These results verify optimal particle dispersion while creating acentrifuge cake that plastically deforms. The cake had a viscosity of670 cP at 100 sec⁻¹.

What is claimed is:
 1. A slurry for the centrifugal classification ofcolloidal particles, comprising:(a) particulate, non-clay, ceramiccolloidal particles: and (b) a dispersant present in substantial excessof that amount necessary for dispersion of the particles and in anamount effective to result in a plastic cake of classified particleswhen the slurry is centrifugally classified.
 2. A slurry according toclaim 1, wherein the particles are selected from the group consistingof: alumina, yttria, silicon nitride, forsterite, mullite, cordierite,and mixtures thereof.
 3. A slurry according to claim 2 wherein theparticles are alumina.
 4. A slurry according to claim 1 where thedispersant is an acrylate copolymer.
 5. A slurry according to claim 4wherein the dispersant is an ammonium polyacrylate polyelectrolyte.
 6. Aslurry according to claim 1 wherein the dispersant is present in anamount of 20-30 weight percent.